This invention is directed to sealants, and more particularly to a method and an apparatus capable of producing custom-colored silicone sealants.
Silicone sealants typically contain a polydiorganosiloxane, a filler, a cross-linker, and a curing catalyst. These silicone sealants cure by exposure to moisture, and are viscous materials which can be extruded from cartridges into cracks or crevices to be sealed. The consistency of a sealant is viscous and toothpaste-like. In applications such as building construction, sealants are thixotropic and non-sagging in order to remain in place until cured. Because silicone sealants are used in building construction, aesthetics such as color are important. Therefore, silicone sealants of a variety of colors are needed commercially, and matching colors for specific building materials can often be a problem.
Silicone sealants are made by mixing various ingredients in predetermined defined weight or volume ratios. For room temperature vulcanizable silicone sealants (RTV), polydiorganosiloxanes are end-blocked with silanol or other appropriate hydrolyzable groups. These polydiorganosiloxanes typically have viscosities in excess of one Pa.s (1,000 centistoke) measured at 25.degree. C., preferably 1 to 100 Pa.s (100,000 centistoke). When a filler is added to the polydiorganosiloxane, the mixture is called a "sealant base", since it constitutes the major portion of the sealant, and because other ingredients are added to arrive at a final composition. Useful fillers are (i) reinforcing fillers such as silica, carbon black, and finely divided calcium carbonates; and (ii) non-reinforcing or extending fillers such as titanium dioxide, quartz, diatomaceous earth, and alumina.
To the "sealant base", cross-linking agents and catalysts are added. The cross-linking agents are generally silanes or partial hydrolysis products of silanes. These silanes include acetoxysilanes, alkoxysilanes, ketoximosilanes, aminosilanes, and amidosilanes. The cross-linking silanes have three to four hydrolyzable groups per molecule, while the partial hydrolysis products have more than three. In addition to cross-linking agents, sealants include chain-extending agents which are also silanes but with only two hydrolyzable groups per molecule. The hydrolyzable group terminating the polydiorganosiloxane is often the same as the group of the silane cross-linking agent, but mixtures of different types of hydrolyzable groups can be present in the same sealant composition.
Catalysts for curing the sealant compositions are dependent upon the type of cross-linking agent, and include compounds such as metal carboxylates, alkyl orthotitanates, titanate chelates, and zirconium alkoxides and chelates.
Since the polydiorganosiloxane in the sealant base is clear and colorless, coloring agents are added. While sealants can be clear, they are usually produced in five to eight standard colors, including black, white, and various tones of beige, brown, or gray. Virtually any color or hue is possible, subject to the reproducibility of the pigment, the exactness of metering, and the thoroughness of mixing. These coloring agents, commonly called pigments, include various categories of inorganic and organic pigments.
For example, the most important inorganic coloring agents employed in sealants are derived from iron oxide pigments, such as the yellow, brown, red, and black, iron oxides. Other synthetic inorganic pigments include for example, cadmium orange, chromium oxide green, manganese violet, and molybdate orange. Typical of numerous varieties of organic synthetic coloring agents for sealants are Acid Red 52, Benzidine Yellow HR, Methyl Violet, Phthalocyanine Green and Blue, Pigment Brown 28, and Victoria Blue B.
To facilitate processing, the coloring agent is added to the "sealant base" in the liquid state. These pigment dispersions, color concentrates, and liquid colorants, are achieved by dispersing a pigment in a liquid carrier.
In the past, processes for coloring sealants have been complicated by the fact that frequent changeovers from color to color are required. Thus, the "sealant base" was compounded, catalyzed, and packaged in drums or bulk containers. The containers were moved to a separate color production process area for pigmentation. Many different color pigments are needed to make the variety of colors and shades required in the market place. Therefore, matching a particular color would often require sophisticated blending of a number of pigments.
Typically, production personnel loaded the "sealant base" into a large mixer, and metered the necessary coloring agent into the mixer to produce the colored sealant. The ingredients were mixed for a period of time, and inspected for color match using standard colorimetry technology. The addition of the pigment had to be carefully controlled, because pigments can cause degradation of the physical properties of the sealant, if the amount of pigment exceeds certain concentration levels. When the color of the sealant was determined to be correct, the sealant was then moved to a proportioning machine where the sealant was dosed into cartridges, drums, or pails, in a one-part volumetric dosing system.
The problem to be solved by the present invention involves the simplification of these complicated prior procedures, and the problem has been solved according to the invention, by the provision of a new automated "hands-off", "on-line", apparatus and method, for producing color matched silicone sealants on demand.